Negative Stiffness Device for Seismic Protection of Structures
Sarlis, Apostolos A.
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Structural weakening and addition of damping is an approach previously proposed for the reduction of seismic forces and drifts in the retrofit of structures. It is also used in the design of new buildings with damping systems. While this approach is efficient, it does not significantly reduce and may even amplify inelastic excursions and permanent deformations of the structural system during a seismic event. This presentation describes the development and operation of a novel Negative Stiffness Device (NSD) that can emulate weakening of the structural system without inelastic excursions and permanent deformations. The NSD simulates yielding by engaging at a prescribed displacement and by applying a force at its installation level that opposes the structural restoring force. The NSD consists of (a) a self contained highly compressed spring in a double negative stiffness magnification mechanism and (b) a "gap spring assembly" (GSA) mechanism which delays the engagement of negative stiffness until the structural system undergoes a prescribed displacement. The NSD employs double chevron braces that self-contain the large vertical forces needed for the development of the horizontal negative stiffness without transferring these forces to the structure. Moreover, this presentation discusses the results from the shake table testing of a 3-story seismically isolated structure equipped with NSD and viscous dampers. These results represent proof of concept for weakening with the use of the NSD. The NSD is shown to have a significant effect on the superstructure response by reducing floor accelerations, story drift and the base shear, and upon the addition of dampers, also results in a reduction in isolator displacements. Finally, the experimental results are compared with analytical results in order to validate the developed analytical and computational models.